[format.string.escape] Fix mis{sing,escaped} braces

This was an oversight in the application of
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2713r0.html
in e38ea31.

Author: tkoeppe

source/utilities.tex

@@ -17154,8 +17154,8 @@
                                                     // \tcode{s4} has value: \tcode{["\textbackslash u\{0\} \textbackslash n \textbackslash t \textbackslash u\{2\} \textbackslash u\{1b\}"]}
 string s5 = format("[{:?}]", "\xc3\x28");           // invalid UTF-8, \tcode{s5} has value: \tcode{["\textbackslash x\{c3\}("]}
 string s6 = format("[{:?}]", "@\importexample{example_02}@");                 @\kern0.75pt@// \tcode{s6} has value: \tcode{["\importexample{example_03}\textbackslash{u}\{200d\}\importexample{example_04}\textbackslash{u}\{fe0f\}"]}
-string s7 = format("[{:?}]", "\u0301");             // \tcode{s7} has value: \tcode{["\textbackslash u{301}"]}
-string s8 = format("[{:?}]", "\\\u0301");           // \tcode{s8} has value: \tcode{["\textbackslash \textbackslash \textbackslash u{301}"]}
+string s7 = format("[{:?}]", "\u0301");             // \tcode{s7} has value: \tcode{["\textbackslash u\{301\}"]}
+string s8 = format("[{:?}]", "\\\u0301");           // \tcode{s8} has value: \tcode{["\textbackslash \textbackslash \textbackslash u\{301\}"]}
 string s9 = format("[{:?}]", "e\u0301\u0323");      // \tcode{s9} has value: \tcode{["\importexample[-2pt]{example_06}"]}
 \end{codeblock}
 \end{example}